Oh! Be A Fine Girl — Kiss Me!

This phrase is a mnemonic device used by students and astronomers to
learn the spectral classification of stars developed by Annie Jump
Cannon.

So what does this classification sequence mean?

It is mainly based on temperature. The O stars are the bluest and
hottest stars, and the M stars are the coolest red stars. The stars
are the beginning of the sequence are traditionally called
“early-type” stars and stars at the end are called “late type”
stars. With in each type there are subdivisions by taking the letter
and adding a decimal digit, such as B0, B1, ... B9 stars. As we know from
“Understanding Her Work”
different temperatures cause the distinction
in star spectra due to electrons occupying different orbitals in atoms
of the atmospheres of the stars. Patterns of spectral lines
are also related to the composition of the stars and can be quite
complicated.
The following two tables summarize the main differences among the
spectra and stars representative of the different spectral classes: